There’s a setting on many digital cameras called auto exposure bracketing, or AEB. When switched into this mode, the camera will shoot three or five or more shots—sometimes with a single press of the button—automatically changing the exposure between each frame. Why do I want three frames instead of one? Because with auto exposure bracketing, the camera doesn’t just deliver three frames, it delivers three different frames.
Depending on the camera model, auto exposure bracketing may be enabled on a menu, or by looking under drive modes, or even with a dedicated AEB button. Often the camera will prompt you to choose how many frames (three or five, for instance) and even how much to change the exposure between each frame. So you could, say, set the camera to shoot at the exposure you believe to be correct, and then one exposure that’s a half-stop underexposed, followed by one exposure that’s a half-stop overexposed.
Back in the good ol’ days before RAW capture, exposure bracketing was a staple of photography—particularly when shooting film. Because you couldn’t verify the accuracy of your exposure until the film was processed, you’d often shoot additional over- and/or under-exposed frames in order to cover your bases just in case. This is called bracketing.
In manual exposure mode, auto exposure bracketing will typically change the shutter speed by the value you’ve selected. For instance, most cameras will allow you to set the exposure change in third-stop increments. So in practice, setting a three-frame bracket at one-third of a stop each time will shoot one frame at the “correct” exposure, then a third-stop underexposed (a slightly faster shutter speed) and a third-stop overexposed (a slightly slower shutter speed). Or you could set it to do two-thirds under and over, a full stop under and over, and so on. You can often adjust the bracket up to three stops difference per frame and to shoot as many as nine total frames.
With auto exposure modes, the camera will defer to your preferences, so in Shutter Priority mode (where you establish the shutter speed) the camera will retain that preferred shutter speed and adjust the aperture to change the exposure. In Aperture Priority, the camera will retain your set ƒ-stop and change the shutter speed to change the exposure.
Your drive and focus settings will also determine whether you need to press the shutter release one time for all three, five, seven or nine shots, or whether a subsequent press of the shutter release will be required for each new exposure. This is adjusted by switching from single-servo autofocus to continuous and/or by choosing high-speed frame advance (as opposed to single-shot drive mode).
So now that you know how auto exposure bracketing works, let’s consider why you might want to use it—a popular question, particularly among photographers who are already capturing increased exposure latitude from RAW image files.
First and foremost, you might want to use AEB to make bigger exposure jumps of 1-, 2- and 3-stops per frame if you’re shooting in tricky lighting situations. For example, let’s say when the entire scene is comprised of especially light or dark tones, where your camera’s auto exposure capability is more likely to be fooled. Likewise, if you’re shooting in a strongly backlit scenario, or where the light is simply behaving in a challenging way, using auto exposure bracketing is a great “cover your behind” approach to ensure a more accurate exposure so you won’t have to push the latitude of your raw file during processing. A good rule of thumb is the more dramatic the lighting, the larger your exposure brackets should be.
Another reason for employing auto exposure bracketing is to make lots of exposures (5-, 7- or 9-frame brackets) if you’re making a high dynamic range (HDR) composite. As great as digital image files are, one of their greatest limitations is lack of dynamic range compared to the human eye. (Think of dynamic range as the number of distinct tones visible between highlight and shadow.) Cameras are producing better dynamic range than ever, but even 12 stops of range pales in comparison to the 20+ stops the human eye can interpret. In an effort to create greater tonal range in digital image files, photographers have long composited multiple exposures into high dynamic range files.
Think about it this way: when you capture an overexposed image, you get more details in the shadows but blow out the highlights. In an underexposed image, you keep highlight detail at the cost of blocking up shadows. But by combining these two (or more) files, you’ll get the best of both worlds—greater details and range in both the highlights and shadows. Some cameras can do this HDR capture and composite automatically, and it starts with an auto exposure bracketing series of shots. But if you’re interested in making a high-dynamic range composite on your own, automatic exposure bracketing goes a long way to simplifying this. By shooting more frames, you’ll get more information captured in the image files, including maintaining details in the brightest highlights and darkest shadows. So by shooting several bracketed frames, you’re maximizing the information available for making a better HDR image file.